ci: Disable eager upgrades from Homebrew

GitHub and Azure's images install a pre-existing Python, which Homebrew
attempts to upgrade if any of the packages we want to install depend on
it.

Unfortunately, this may cause conflicts on Python's symlinks, so stop
Homebrew from trying to do an automatic upgrade.

Author: QuLogic

.github/workflows/tests.yml

@@ -158,6 +158,7 @@ jobs:
             ;;
           macOS)
             brew update
+            export HOMEBREW_NO_INSTALL_UPGRADE=1
             brew install ccache ghostscript ninja
             # The macOS 12 images have an older Python, and this causes homebrew to generate conflicts.
             # We'll just skip GTK for now, to not pull in Python updates.

azure-pipelines.yml

@@ -111,6 +111,7 @@ stages:
                 ;;
               Darwin)
                 brew update
+                export HOMEBREW_NO_INSTALL_UPGRADE=1
                 brew install --cask xquartz
                 brew install ccache ffmpeg imagemagick mplayer ninja pkg-config
                 brew install --cask font-noto-sans-cjk-sc